You can use a LEFT JOIN between the Date table and the source table. This will fill in missing dates with zeros for Sales and Order_Quantity .

SELECT COALESCE(S.Sales, 0) AS Sales, COALESCE(S.Order_Qunty, '0000') AS Order_Qunty, D.Date
FROM DateTable D
LEFT JOIN SourceTable S ON D.Date = S.Date
WHERE D.Date BETWEEN '20230101' AND '20231212' -- Date range from the SourceTable
ORDER BY D.Date;

This is implementation of PostgreSQL.

WITH date_sequence AS (
    SELECT generate_series(
        (SELECT MIN(date_column) FROM your_existing_data), 
        CURRENT_DATE, 
        '1 day'::interval
    )::DATE AS date_column
)
SELECT COALESCE(your_existing_data.Sales, 0) AS Sales, COALESCE(your_existing_data.Order_Qunty, '0000') AS Order_Qunty, ds.date_column
FROM date_sequence ds
LEFT JOIN your_existing_data ON ds.date_column = your_existing_data.date_column
ORDER BY ds.date_column;

In SqlServer

WITH DateSequence AS (
    SELECT MIN(date_column) AS date_column
    FROM your_existing_data
    
    UNION ALL
    
    SELECT DATEADD(DAY, 1, date_column)
    FROM DateSequence
    WHERE DATEADD(DAY, 1, date_column) <= GETDATE()
)
SELECT COALESCE(your_existing_data.Sales, 0) AS Sales, COALESCE(your_existing_data.Order_Qunty, '0000') AS Order_Qunty, ds.date_column
FROM DateSequence ds
LEFT JOIN your_existing_data ON ds.date_column = your_existing_data.date_column
ORDER BY ds.date_column;

Please replace your_existing_data to your real table name.